A major problem associated with electrical distribution cable is its tendency, over a period of time, to fail due to the progressive degradation of its insulation. The degradative processes involved in the failure of cables are correlated with two "treeing" phenomena. "Electrical treeing" is the product of numerous electrical discharges in the presence of strong electrical fields which eventually lead to the formation of voids within the insulation material. These voids resemble the trunk and branches of a tree in profile under microscopic observation, from which the descriptive terminology derives. As the trees formed by this process grow, they provide further routes along which corona discharges can occur, the cumulative effect being electrical breakdown of the insulation. Electrical treeing generally occurs when large voltages are imposed on the cable. The degradative results of the electrical treeing process can be precipitous such that the electrical cable can break down in a relatively short period of time.
The second type of treeing, known as "water treeing," is observed when the insulation material is simultaneously exposed to moisture and an electric field. This mechanism is much more gradual than electrical treeing, requiring an extended period of time to cause the degree of damage that affects the insulation characteristics of the distribution cable. However, since water treeing occurs at considerably lower electrical fields than required for the formation of electrical trees, this phenomenon is a leading cause of reduced service life of cables which allow water entry to the conductor region, whether through diffusion or some other mechanism.
Efforts have been made to prepare cables which are resistant to water treeing, an example of this approach being the teachings of Bahder in U.S. Pat. No. 4,354,992. Here, a method of fabricating a cable is described wherein a particular extrusion sequence allows a swelling of the insulation which results in a reduced number and size of voids which are precursors of trees. In addition. pressure curing the cable above its melting point followed by cooling under pressure was found to further improve resistance to trees.
In another approach, cable deterioration is minimized by limiting the amount of water which can enter the cable interior. Examples of this technique can be illustrated by U.S. Pat. No. 3,252,834 to Vincent, wherein a composition comprising a polyester resin, a silicone resin, an insecticide and a solvent is used to coat the interior of the cable, and U.S. Pat. No. 4,845,309 to Vincent et al., wherein a curable silicone "water block" is pumped into the cable's interior and subsequently crosslinked.
Yet another method used to prevent the degradation of cable insulation comprises filling the cable interior with an oil. Thus, U.S. Pat. No. 3,527,874 to Hayami teaches the use of silicon (sic) oil, or a silicon-hydrocarbon oil mixture, to fill the interstices between the conductor and insulation of an electrical distribution cable. Hayami teaches using low viscosity oils which can flow or exude through the cable's insulation layer. Thus the positive effects of the oil on the insulation would be lost when the oil leaks or exudes from the cable interior.
It is also known that water treeing can be reduced by the incorporation of an anti-tree additive (e.g., various organo silanes) directly into the insulation composition of the cable at time of manufacture.
The above described methods only address the issue of how to inhibit the formation of trees and the associated deterioration of insulation integrity. It will, however, be recognized that a vast network of underground cable is already in place wherein the cable either has not been treated according to one of these procedures or it has degenerated significantly despite such efforts and is therefore subject to premature failure. As a partial answer to industry's desire to extend the useful life of existing underground cables, it has been found that certain tree retardants can be introduced into the cable's interior to partially restore the insulation performance. An example of such a cable reclamation technique is found in U.S. Pat. No. 4,372,988 to Bahder. This patent teaches a method for reclaiming electrical distribution cable which comprises' purging the cable with a desiccant gas: then supplying, in a continuous fashion, a tree retardant liquid, such as polydimethylsiloxane fluid, to the interior of the cable. This disclosure also suffers from an above mentioned disadvantage in that the fluid can exude or leak from the cable, and this reference addresses the potential loss of fluid by providing reservoirs which can maintain a constant fluid level, further adding to the complexity of this system. A similar method was proposed by Vincent et al. in U.S. Pat. No. 4,766,011, wherein the tree retardant fluid was selected from a particular class of aromatic alkoxysilanes. Again, the tree retardant fluid was supplied to the interstices of the conductor of the cable. However, fluid of this prior art disclosure could be polymerized within the water tree voids in the insulation and therefore would not leak out of the cable.
Such reclamation methods are effective to an extent, but typically require complicated and specialized equipment to pump the restorative fluid into the relatively tight confines of the cable's interior, even when the fluid has a very low viscosity. The result is that a contractor often finds it economically equivalent, or even advantageous, to completely replace a cable once it has deteriorated rather than avail himself of one of the above restorative methods.